Methods for providing lithography features on a substrate by self-assembly of block copolymers

ABSTRACT

A method of forming at least one lithography feature, the method including: providing at least one lithography recess on a substrate, the or each lithography recess having at least one side-wall and a base, with the at least one side-wall having a width between portions thereof; providing a self-assemblable block copolymer having first and second blocks in the or each lithography recess; causing the self-assemblable block copolymer to self-assemble into an ordered layer within the or each lithography recess, the ordered layer including at least a first domain of first blocks and a second domain of second blocks; causing the self-assemblable block copolymer to cross-link in a directional manner; and selectively removing the first domain to form lithography features of the second domain within the or each lithography recess.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of EP application 14170406, whichwas filed on May 28, 2014 and which is incorporated herein in itsentirety by reference.

FIELD OF INVENTION

The present invention relates to methods of forming lithography featureson a substrate, by use of self-assembly of block copolymers in a recessprovided on the substrate. The methods may be useful for forming contactholes providing access between layers of semiconductor devices.

BACKGROUND

In lithography for device manufacture, there is an ongoing desire toreduce the size of features in a lithographic pattern in order toincrease the density of features on a given substrate area. Patterns ofsmaller features having critical dimensions at nano-scale allow forgreater concentrations of device or circuit structures, yieldingpotential improvements in size reduction and manufacturing costs forelectronic and other devices. In projection photolithography, the pushfor smaller features has resulted in the development of technologiessuch as immersion lithography and extreme ultraviolet (EUV) lithography.

As an alternative, so-called imprint lithography generally involves theuse of a “stamp” (often referred to as an imprint template) to transfera pattern onto a substrate. An advantage of imprint lithography is thatthe resolution of the features is not limited by, for example, theemission wavelength of a radiation source or the numerical aperture of aprojection system. Instead, the resolution is mainly limited to thepattern density on the imprint template.

For both projection photolithography and for imprint lithography, it isdesirable to provide high resolution patterning of surfaces, either ofimprint templates or of other substrates. The use of self-assembly ofblock copolymers (BCPs) has been considered as a potential method forincreasing the feature resolution to smaller dimensions than thoseobtainable by prior art lithography methods or as an alternative toelectron beam lithography for preparation of imprint templates.

BCPs comprise different blocks, each comprising identical monomers, andarranged side-by side along the polymer chain. Each block may containmany monomers of its respective type. So, for instance, an A-B blockcopolymer may have a plurality of type A monomers in the (or each) Ablock and a plurality of type B monomers in the (or each) B block. Anexample of a suitable BCP is, for instance, a polymer having covalentlylinked blocks of polystyrene (PS) monomers (hydrophobic block) andpolymethylmethacrylate (PMMA) monomers (hydrophilic block). Other BCPswith blocks of differing hydrophobicity/hydrophilicity may be useful.For instance tri-block copolymers (A-B-C) may also be useful, as mayalternating or periodic block copolymers (e.g. [-A-B-A-B-A-B-]_(n) or[-A-B-C-A-B-C]_(m) where n and m are integers). The blocks are connectedto each other by covalent links in a linear or branched fashion (e.g.star or branched configuration).

Self-assemblable BCPs are compounds useful in nanofabrication becausethey may undergo an order-disorder transition on cooling below a certaintemperature (order-disorder transition temperature To/d) resulting inphase separation of copolymer blocks of different chemical nature toform ordered, chemically distinct domains with dimensions from tens ofnanometers to dimensions which are below 10 nm. The size and shape ofthe domains may be controlled by manipulating the molecular weight andcomposition of the different block types of the copolymer. Theinterfaces between the domains may have line width roughness of theorder of 1-5 nm and may be manipulated by modification of the chemicalcompositions of the blocks of the copolymers.

The feasibility of using thin films of BCPs as self-assembling templateswas demonstrated by Chaikin and Register, et al., Science 276, 1401(1997). Dense arrays of dots and holes with dimensions of 20 nm weretransferred from a thin film of poly(styrene-block-isoprene) to siliconnitride substrates.

BCPs may form many different phases upon self-assembly, dependent uponthe volume fractions of the blocks, degree of polymerization within eachblock type (i.e. number of monomers of each respective type within eachrespective block), the optional use of solvents and surfaceinteractions. When applied in thin films, geometric confinement may poseadditional boundary conditions that may limit the phases formed. Ingeneral spherical (e.g. cubic), cylindrical (e.g. tetragonal orhexagonal) and lamellar phases (i.e. self-assembled phases with cubic,hexagonal or lamellar space-filling symmetry) are practically observedin thin films of self-assembled BCPs.

The phase type observed may depend upon the relative molecular volumefractions of the different polymer blocks. For instance, a molecularvolume ratio of 80:20 will provide a cubic phase of discontinuousspherical domains of the low volume block arranged in a continuousdomain of the higher volume block. As the volume ratio reduces to 70:30,a cylindrical phase will be formed with the discontinuous domains beingcylinders of the lower volume block. At 50:50 ratio a lamellar phase isformed. With a ratio of 30:70 an inverted cylindrical phase may beformed and at a ratio of 20:80, an inverted cubic phase may be formed.

Suitable BCPs for use as self-assemblable polymers include, but are notlimited to, poly(styrene-b-methylmethacrylate),poly(styrene-b-2-vinylpyridone), poly(styrene-b-butadiene),poly(styrene-b-ferrocenyldimethylsilane), poly(styrene-b-ethyleneoxide),poly(ethyleneoxide-b-isoprene). The symbol “b” signifies “block”Although these are di-block copolymers as examples, it will be apparentto the skilled person that self-assembly may also employ tri-block,tetra-block or other multi-block copolymers.

One prior art method used to guide or direct self-assembly of polymers(such as BCPs) onto substrate surfaces is known as graphoepitaxy. Thismethod involves the self-organization of BCPs guided by topologicalpre-patterning on the substrate using features constructed of resist (orfeatures transferred from resist onto a substrate surface, or featurestransferred onto film stacks deposited on the substrate surface). Thepre-patterning is used to form an enclosure or “recess” comprising asubstrate base and side-walls of resist (or side-walls formed in a filmor side-walls formed in the substrate).

Typically, the height of features of a graphoepitaxy template is of theorder of the thickness of the BCP layer to be ordered, so may be, forinstance, from about 20 nm to about 150 nm.

Lamellar self-assembled BCPs can form parallel linear patterns oflithography features with adjacent lines of the different polymer blockdomains in the recesses. For instance if the BCP is a di-block copolymerwith A and B blocks within the polymer chain, the BCP may self-assembleinto an ordered layer in each recess, the layer comprising regularlyspaced first domains of A blocks, alternating with second domains of Bblocks.

Similarly, cylindrical self-assembled BCPs can also form patterns oflithography features comprising cylindrical discontinuous first domainssurrounded by a second continuous domain. For instance, if the BCP is adi-block copolymer with A and B blocks within the polymer chain, the Ablocks may assemble into a cylindrical discontinuous domain within acircular recess and surrounded by a continuous domain of B blocks.Alternatively, the A blocks may assemble into cylindrical discontinuousdomains regularly spaced across a linear recess and surrounded by acontinuous domain of B blocks.

Graphoepitaxy may be used, therefore, to guide the self-organization oflamellar or cylindrical phases such that the BCP pattern subdivides thespacing of the side walls of a recess into domains of discrete copolymerpatterns.

In a process to implement the use of BCP self-assembly innanofabrication, a substrate may be modified with a neutral orientationcontrol layer, as part of the graphoepitaxy template, to induce thepreferred orientation of the self-assembly pattern in relation to thesubstrate. For some BCPs used in self-assemblable polymer layers, theremay be a preferential interaction between one of the blocks and thesubstrate surface that may result in orientation. For instance, for apolystyrene (PS)-b-PMMA block copolymer, the PMMA block willpreferentially wet (i.e. have a high chemical affinity with) oxidesurfaces and this may be used to induce the self-assembled pattern tolie oriented parallel to the plane of the surface. Normal orientationmay be induced, for instance, by depositing a neutral orientation layeronto the surface rendering the substrate surface neutral to both blocks,in other words the neutral orientation layer has a similar chemicalaffinity for each block, such that both blocks wet the neutralorientation layer at the surface in a similar manner. By “normalorientation” it is meant that the domains of each block will bepositioned side-by-side at the substrate surface, with the interfacialregions between adjacent domains of different blocks lying substantiallyperpendicular to the plane of the surface.

In a graphoepitaxy template for aligning a di-block copolymer having Aand B blocks, where A is hydrophilic and B is hydrophobic in nature, thegraphoepitaxy pattern may comprise hydrophobic resist side-wallfeatures, with a neutral orientation base between the hydrophobic resistfeatures. The B domains may preferentially assemble alongside thehydrophobic resist features, with several alternating domains of A and Bblocks aligned over the neutral orientation regions between the pinningresist features of the graphoepitaxy template.

Neutral orientation layers may be created by, for instance, use ofrandom copolymer brushes which are covalently linked to the substrate byreaction of a hydroxyl terminal group, or some other reactive end group,with oxide at the substrate surface. In other arrangements for neutralorientation layer formation, crosslinkable random copolymers orappropriate silanes (i.e. molecules with a substituted reactive silanesuch as a (tri)chlorosilane or (tri)methoxysilane, also known as silyl,end group) may be used to render surfaces neutral by acting as anintermediate layer between the substrate surface and the layer ofself-assemblable polymer. Such silane based neutral orientation layerswill typically be present as a monolayer whereas crosslinkable polymersare typically not present as a monolayer, and may have a layer thicknessof typically less than about 20 nm.

A thin layer of self-assemblable BCP may be deposited onto a substrate,having a graphoepitaxy template as set out above. A suitable method fordeposition of the self-assemblable polymer is spin coating, as thisprocess is capable of providing well defined, uniform, thin layers ofself-assemblable polymer. A suitable layer thickness for depositedself-assemblable polymer films is approximately about 10 to about 150nm.

Following deposition of the BCP film, the film may still be disorderedor only partially ordered and additional steps may be needed to promoteand/or complete self-assembly. For instance, the self-assemblablepolymer may be deposited as a solution in a solvent, with solventremoval, for instance by evaporation, required prior to self-assembly.

Self-assembly of BCPs is a process where the assembly of many smallcomponents (the BCPs) results in the formation of larger more complexstructures (the nanometer sized features in the self-assembled pattern,referred to as domains in this specification). Defects arise naturallyfrom the physics controlling the self-assembly of the polymers.Self-assembly is driven by the differences in interactions (i.e.differences in mutual chemical affinity) between A/A, B/B and A/B (orB/A) block pairs of an A-B block copolymer, with the driving force forphase separation described by Flory-Huggins theory for the system underconsideration. The use of graphoepitaxy may greatly reduce defectformation. The Flory-Huggins interaction parameter (chi value), and thedegree of polymerisation of the BCP blocks (N value) are parameters ofthe BCPs which affect the phase separation, and the dimensions withwhich self-assembly of a particular BCP occurs.

For polymers which undergo self-assembly, the self-assemblable polymerwill exhibit an order-disorder temperature To/d. To/d may be measured byany suitable technique for assessing the ordered/disordered state of thepolymer, such as differential scanning calorimetry (DSC). If layerformation takes place below this temperature, the molecules will bedriven to self-assemble. Above the temperature To/d, a disordered layerwill be formed with the entropy contribution from disordered A/B domainsoutweighing the enthalpy contribution arising from favourableinteractions between neighbouring A-A and B-B block pairs in the layer.The self-assemblable polymer may also exhibit a glass transitiontemperature Tg below which the polymer is effectively immobilized andabove which the copolymer molecules may still reorient within a layerrelative to neighbouring copolymer molecules. The glass transitiontemperature is also suitably measured by DSC.

Defects formed during ordering as set out above may be partly removed byannealing. Defects such as disclinations (which are line defects inwhich rotational symmetry is violated, e.g. where there is a defect inthe orientation of a director) may be annihilated by pairing with otherdefects or disclinations of opposite sign. Chain mobility of theself-assemblable polymer may be a crucial factor for determining defectmigration and annihilation and so annealing may be carried out attemperatures where chain mobility is high but the self-assembled orderedpattern is not lost. This implies temperatures up to a few ° C. belowthe order/disorder temperature To/d for the polymer.

Ordering and defect annihilation may be combined into a single annealingprocess or a plurality of processes may be used in order to provide alayer of self-assembled polymer such as BCP, having an ordered patternof domains of differing chemical types (i.e. of domains of differentblock types).

In order to transfer a pattern, such as a device architecture ortopology, from the self-assembled polymer layer into the substrate uponwhich the self-assembled polymer is deposited, typically a first domaintype will be removed by so-called breakthrough etching to provide apattern of a second domain type on the surface of the substrate with thesubstrate laid bare between the lithography features of the seconddomain type. Patterns having parallel cylindrical phase domains can beetched using dry etching or reactive ion etching techniques. Patternshaving lamellar phase domains can utilise wet etching techniques inaddition to those suitable for the etching of parallel cylindrical phasedomains.

Following the breakthrough etching, the pattern of the ordered BCP maybe transferred by so-called transfer etching using an etching meanswhich is resisted by the second domain type and so forms recesses in thesubstrate surface where the surface has been laid bare.

Spacing between lithography features is known as pitch—defined as thewidth of one repeat unit of the lithography feature (i.e. feature widthplus inter-feature spacing). Self-assembly processes using BCPs can beused to produce lithography features with particularly low pitch,typically less than 30-50 nm.

Self-assembly of BCPs is also controlled by the spacing of photo-resistwalls and the BCP material thickness. The thickness of the BCP layerwithin a graphoepitaxy template may be optimised for the formation ofdistinct domains of type A and type B polymers within regions of thegraphoepitaxy template. The placement of the domains of type A and typeB polymers within regions of the graphoepitaxy template may be guided bythe arrangement of the graphoepitaxy template.

For example, a circular recess may be defined on a substrate surface. Adeposited BCP layer may be caused to self-assemble within the circularrecess to form distinct domains of polymers. A first type A polymerdomain may be formed as a cylinder within a continuous type B polymerdomain within the recess. Breakthrough etching may be used to remove thecylindrical type A polymer domain, resulting in the formation of acircular opening. The circular opening may be centrally located withinthe circular recess and may allow further processing to be carried outon the substrate, such as, for example, etching of the substrate in theregion of the circular opening. It will be appreciated that theplacement of the opening with respect to the placement of the recesscontrols the accuracy of the placement of the further processing carriedout on the substrate.

It would be useful to be able to construct multiple BCP features on asubstrate with a substantially predictable placement.

It is an object of the invention to obviate or mitigate one or moredisadvantage associated with the prior art.

SUMMARY OF INVENTION

In accordance with the first aspect of the present invention, there isprovided a method of forming at least one lithography feature, themethod comprising: providing at least one lithography recess on asubstrate, the or each lithography recess comprising side-walls and abase, with the side-walls having a width therebetween; providing aself-assemblable block copolymer having first and second blocks in theor each lithography recess; causing the self-assemblable block copolymerto self-assemble into an ordered layer within the or each lithographyrecess, the ordered layer comprising at least a first domain of firstblocks and a second domain of second blocks; causing theself-assemblable block copolymer to cross-link in a directional manner;and selectively removing the first domain to form lithography featurescomprised of the second domain within the or each lithography recess.

Causing the self-assemblable block copolymer to cross-link in adirectional manner may be interpreted as meaning causing thecross-linking to initiate at a first location and causing thecross-linking to proceed towards a second location. Cross-linking of ablock-copolymer in a directional manner allows the placement of domainsof the first blocks and the second blocks to be controlled accurately,so as to accurately position the domains (and hence the lithographyfeatures) within the lithography recess.

The following features are applicable to the invention whereappropriate. When suitable, combinations of the following features maybe employed as part of the invention, for instance as set out in theclaims. The invention is particularly suitable for use in devicelithography. For instance, the invention may be of use in patterning asubstrate which is used to form a device, or may be of use in patterningan imprint template for use in imprint lithography (which may then beused to form devices).

The cross-linking may be initiated at the side-walls and proceed awayfrom the sidewalls. The sidewalls provide a convenient location at whichto initiate the cross-linking. For example the cross-linking may beinitiated by a layer provided on the sidewall, or by a materialcontained within the side-walls.

The self-assemblable block copolymer may be caused to self-assembleduring an annealing process. The self-assemblable block copolymer may becaused to cross-link during an annealing process.

Cross-linking during an annealing process may allow a block copolymer toboth self-assembly and cross-link during a single processing step,providing a simple process through which cross-linking can be broughtabout.

The block copolymer may comprise polystyrene blocks. The polystyreneblocks may comprise glycidyl moieties. Providing glycidyl moietieswithin the polystyrene blocks allows the polystyrene blocks to becross-linked by polymerisation of the glycidyl moieties. The glycidylmoieties can readily be caused to polymerise by an acid catalyst.

The polystyrene blocks may comprise more than about 0.1% by weight ofglycidyl moieties. The polystyrene blocks may comprise less than about10% by weight of glycidyl moieties. Preferably the polystyrene blockscomprise more than or equal to about 1% by weight of glycidyl moieties.Preferably the polystyrene blocks comprise less or equal to about 2% byweight of glycidyl moieties.

Causing the self-assemblable block copolymer to cross-link may beinitiated by an acid. The acid may be generated by a photo-acidgenerator. The acid may be at least partially generated by a photo-acidgenerator during a lithography exposure.

A lithography exposure is an exposure of a substrate to a patternedradiation beam, generally used to transfer a pattern to the substrate.For example, the pattern transferred may be that required to define thelithography recess. The generation of an acid during the lithographyexposure is common where chemically amplified resists are used toincrease the sensitivity of a photo-resist. Using an acid alreadygenerated within a resist to initiate cross-linking allows the acidwhich is already present (and which has served its primary purpose) tobe used for a secondary purpose (i.e. initiating cross-linking).

The acid may be at least partially generated by a photo-acid generatorduring a flood exposure. Rather than, or in addition to, a patternedlithography exposure, a flood exposure allows an entire substrate to beexposed to radiation. This can allow a photo-acid generator which hasnot previously been activated to be activated and to generate an acid.

The recesses may be formed in resist. The photo-acid generator may beprovided within the resist.

The photo acid generator may be provided on the side-walls of the atleast one lithography recess.

The recesses may be formed in the substrate.

The or each lithography recess may be circular. The self-assemblableblock copolymer may be adapted to form an ordered layer havingcylindrical first domains of the first blocks in a cylindricalarrangement surrounded by a second continuous domain of the secondblocks, the cylindrical first domain being oriented perpendicular to thesubstrate. The use of a circular lithography recess allows thedefinition of circular lithography features.

Contact holes may be circular openings which allow access betweennon-adjacent layers on a substrate. A contact hole is an example of alithography feature. The use of self-assembly of BCPs in a lithographyrecess to form a contact hole may allow a hole to be formed having asmaller lateral dimension than the dimensions of the lithography recess.The application of this self-assembly technique to the formation ofcontact holes provides the advantage of reducing the dimension of thecontact hole.

The or each lithography recess may be linear. The self-assemblable blockcopolymer may be adapted to form a lamellar ordered layer wherein thefirst domains are lamellae alternating with second domains which arealso lamellae, the lamellae of the first and second domains beingorientated with their planar surfaces lying perpendicular to thesubstrate and parallel to the recess walls. The use of a linearlithography recess allows the definition of linear lithography features.

The cross-linking may be initiated at the side-walls and proceed towardsthe centre of the recess.

The or each lithography recess may be used to form contact holes.

The lithography features may have a minimum lateral dimension of about40 nm or less. The lithography features may have a minimum lateraldimension of about 5 nm or more. The lithography features formed by theself-assembly of BCPs may allow the definition of smaller lithographyfeatures than would be defined by conventional lithography methodsalone. Alternatively, the use of self-assembly of BCPs may allow thedefinition of lithography features with more uniformity than would bepossible with lithography features defined by conventional lithographytechniques at such small dimensions.

In order to direct self assembly and reduce defects, the side-walls mayhave a higher chemical affinity for one of the BCPs such that, uponassembly, the BCP having the higher chemical affinity with the side-wallis caused to assemble alongside that side-wall. Chemical affinity may beprovided by utilising hydrophobic or hydrophilic side-wall features.

The step of providing a layer of self-assemblable BCP in the recess maybe carried out by spin coating of a solution of the BCP followed byremoval of solvent.

The self-assemblable BCP may be caused to self-assemble by lowering thetemperature to a temperature less than To/d for the BCP, to give anordered layer of self-assembled BCP in the recess.

The substrate may be a semiconductor substrate, and may comprise aplurality of layers forming the substrate. For instance, the outermostlayer of the substrate may be an ARC (anti-reflection coating) layer.

The outermost layer of the substrate may be neutral to the domains ofthe BCP, by which it is meant that it has a similar chemical affinityfor each of the domain types of the BCP. The neutral orientation layermay, for example, be created by use of random copolymer brushes.Alternatively, an orientation control layer may be provided as anuppermost or outermost surface layer of the substrate to induce apreferred orientation of the self-assembly pattern in relation to thesubstrate.

The recesses comprising side-walls may be formed by photolithography,for instance with actinic radiation such as UV, EUV or DUV (deep UV)radiation.

The recesses may for example be formed in resist. The recesses may forexample be formed on a substrate surface (e.g. having been transferredfrom resist onto the substrate). The recess may for example be formed ina film stack (e.g. having been transferred from resist onto the filmstack).

The height of the recesses may be of the order of the thickness of theBCP layer to be ordered. The height of the recesses may for example befrom about 20 nm to about 150 nm (e.g. about 100 nm).

The step of selectively removing one of the domains may be achieved byetching, wherein the ordered layer of self-assembled BCP acts as aresist layer for etching lithography features within the recess on thesubstrate. Selective etching can be achieved by utilising polymershaving different etch resist properties and by selection of an etchantcapable of selectively etching certain of the polymer domains. Selectiveremoval may alternatively be achieved, for instance, by selectivephoto-degradation or photo-cleavage of a linking agent between blocks ofthe copolymer and subsequent solubilisation of one of the blocks.

According to a second aspect of the invention there is provided a methodfor forming a device topography at a surface of a substrate, the methodcomprising using the self-assembled block copolymer layer formed by themethod according to the first aspect of the invention as a resist layerwhilst etching the substrate to provide the device topography.

According to a third aspect of the invention there is provided a methodof forming at least one lithography feature on a substrate, thesubstrate comprising at least one lithography recess, the or eachlithography recess comprising side-walls and a base, with the side-wallshaving a width therebetween, the method comprising: providing aself-assemblable block copolymer having first and second blocks in theor each lithography recess; causing the self-assemblable block copolymerto self-assemble into an ordered layer within the or each lithographyrecess, the ordered layer comprising at least a first domain of firstblocks and a second domain of second blocks; causing theself-assemblable block copolymer to cross-link in a directional manner;and selectively removing the first domain to form lithography featurescomprised of the second domain within the or each lithography recess.

According to a fourth aspect of the invention there is provided alithographic tool comprising: a heat source arranged to anneal asubstrate; and a radiation source arranged to irradiate the substrateduring annealing. The provision of a radiation source within alithographic tool which also comprises a heat source allows a photo acidgenerator to be activated, so as to initiate cross-linking, at alocation which is remote from a lithography exposure location. Forexample, annealing may be carried out on a wafer track while a substrateis transported within a semiconductor fabrication plant.

The radiation source may be arranged to emit actinic radiation. Theradiation source may be arranged to emit UV radiation. The radiationsource may be arranged to emit DUV radiation. The radiation source maybe arranged to emit EUV radiation. A photo acid generator may beactivated by longer wavelength radiation than is required for alithography exposure. Radiation with a wavelength which is longer thanwould be required for a lithography exposure can be used as there is noneed for a pattern to be transferred by the radiation which activates aphoto acid generator.

The heat source may be a hotplate.

The present invention relates to lithography methods. The methods may beused in processes for the manufacture of devices, such as electronicdevices and integrated circuits or other applications, such as themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, flat-panel displays,liquid-crystal displays (LCDs), thin film magnetic heads, organic lightemitting diodes, etc. The invention is also of use to create regularnanostructures on a surface for use in the fabrication of integratedcircuits, bit-patterned media and/or discrete track media for magneticstorage devices (e.g. for hard drives).

The self-assemblable BCP may be a BCP as set out hereinbefore comprisingat least two different block types, referred to as first and secondpolymer blocks, which are self-assemblable into an ordered polymer layerhaving the different block types associated into first and second domaintypes. The BCP may comprise di-block copolymer and/or tri-block ormulti-block copolymers. Alternating or periodic BCPs may also be used inthe self-assemblable BCP.

In an embodiment the cross-linking is covalent bonding between polymerchains. For example, a functional group within a first polymer chainundergoing a chemical reaction so as to polymerise with a similarfunctional group within a second polymer chain is considered to be theformation of a cross-link between the first and second polymer chains.

By chemical affinity, in this specification, is meant the tendency oftwo differing chemical species to associate together. For instancechemical species which are hydrophilic in nature have a high chemicalaffinity for water whereas hydrophobic compounds have a low chemicalaffinity for water but a high chemical affinity for alkanes. Chemicalspecies which are polar in nature have a high chemical affinity forother polar compounds and for water whereas apolar, non-polar orhydrophobic compounds have a low chemical affinity for water and polarspecies but may exhibit high chemical affinity for other non-polarspecies such as alkanes or the like. The chemical affinity is related tothe free energy associated with an interface between two chemicalspecies: if the interfacial free energy is high, then the two specieshave a low chemical affinity for each other whereas if the interfacialfree energy is low, then the two species have a high chemical affinityfor each other. Chemical affinities of surfaces may be measured, forinstance, by means of contact angle measurements using various liquids,so that if one surface has the same contact angle for a liquid asanother surface, the two surfaces may be said to have substantially thesame chemical affinity for the liquid. If the contact angles differ forthe two surfaces, the surface with the smaller contact angle has ahigher chemical affinity for the liquid than the surface with the largercontact angle.

By “chemical species” in this specification is meant either a chemicalcompound such as a molecule, oligomer or polymer, or, in the case of anamphiphilic molecule (i.e. a molecule having at least two interconnectedmoieties having differing chemical affinities), the term “chemicalspecies” may refer to the different moieties of such molecules. Forinstance, in the case of a di-block copolymer, the two different polymerblocks making up the self-assemblable BCP molecule are considered as twodifferent chemical species having differing chemical affinities.

Throughout this specification, the term “comprising” or “comprises”means including the component(s) specified but not to the exclusion ofthe presence of others. The term “consisting essentially of” or“consists essentially of” means including the components specified butexcluding other components except for materials present as impurities,unavoidable materials present as a result of processes used to providethe components, and components added for a purpose other than achievingthe technical effect of the invention. Typically, a compositionconsisting essentially of a set of components will comprise less than 5%by weight, typically less than 3% by weight, more typically less than 1%by weight of non-specified components. The terms “consist of” or“consisting of” mean including the components specified but excludingthe deliberate addition of other components.

Whenever appropriate, the use of the term “comprises” or “comprising”may also be taken to include the meaning “consist of” or “consistingof”, “consists essentially of” or “consisting essentially of”.

In this specification, when reference is made to the thickness of afeature, the thickness is suitably measured by an appropriate meansalong an axis normal to the substrate surface and passing through thecentroid of the feature. Thickness may suitably be measured bytechniques such as interferometry or assessed through knowledge of etchrates.

Wherever mention is made of a “layer” in this specification, the layerreferred to is to be taken to be layer of substantially uniformthickness, where present. By “substantially uniform thickness” is meantthat the thickness does not vary by more than 10%, preferably not morethan 5% of its average value across the layer.

In this specification “recess” is not intended to imply a particularshape. The term “recess” may be interpreted as meaning a lithographyfeature formed on the surface of a substrate, which has a depth andside-walls. A recess may for example be circular in shape, for exampledefining a contact hole, having a diameter or width and havingside-walls which, in cross-section, appear vertical. Alternatively, arecess may be linear in shape, for example defining a trench, havingside-walls which are separated by a width in a first direction, andextend in an elongate manner in a second direction. It will beappreciated that a recess may take any other convenient form, and mayinclude linear or curved sections. A lithography feature may compriseone or more recesses. The term “lithography recess” may be interpretedas meaning a recess which is intended to result in the production of alithography feature.

In this specification, the term “substrate” is meant to include anysurface layers forming part of the substrate, or being provided on asubstrate, such as other planarization layers or anti-reflection coatinglayers which may be at, or form, the surface of the substrate, or mayinclude other layers such as those specifically mentioned above withreference to particular embodiments of the invention.

In this specification, the term “lateral” may be interpreted as meaningin the plane of the surface of a substrate. For example, the width ordiameter of a recess may be considered to be a lateral dimension of thatrecess. Alternatively, the length of a recess may be considered to be alateral dimension of that recess. However, the depth of a recess wouldnot be considered to be a lateral dimension of that recess.

One or more aspects of the invention may, where appropriate to oneskilled in the art, be combined with any one or more other aspectsdescribed herein, and/or with any one or more features described herein.In particular, features described with reference to the first aspect ofthe invention may be combined with the second and/or third aspects ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the invention will be described with referenceto the accompanying Figures, in which:

FIG. 1 schematically depicts directed self-assembly of A-B blockcopolymers onto a substrate by graphoepitaxy;

FIG. 2 schematically depicts directed self-assembly of A-B blockcopolymers onto a substrate by graphoepitaxy according to an embodimentof the invention;

FIG. 3 schematically depicts in more detail the directed self-assemblyof A-B block copolymers according to the embodiment of the inventionshown in FIG. 2;

FIG. 4 schematically depicts directed self-assembly of A-B blockcopolymers onto a substrate by graphoepitaxy according to an alternativeembodiment of the invention; and

FIG. 5 is a schematic representation of a substrate on whichgraphoepitaxy according to an alternative embodiment of the inventionhas been performed.

DETAILED DESCRIPTION

The described and illustrated embodiments are to be considered asillustrative and not restrictive in character, it being understood thatonly preferred embodiments have been shown and/or described and that allchanges and modifications that come within the scope of the inventionsas defined in the claims are desired to be protected.

FIGS. 1A and 1B show, in plan view and cross-section respectively, partof a substrate 1 to which a lithography process using self-assembly ofBCPs is applied. An anti-reflection coating may be present on thesurface of the substrate 1. The anti-reflection coating (if present) maybe an organic material, such as, for example, ARC 29, from BrewerScience. Alternatively, the anti-reflection coating may be an inorganicmaterial such as, for example, SiC or SiON. A layer of photo-resist 2 isapplied to the substrate 1. The layer of photo-resist 2 is patternedwith a plurality of contact hole resist recesses 3.

In FIG. 1C, a BCP layer 4 has been deposited onto the substrate 1 andthe photo-resist 2. The BCP layer 4 is shown with a uniform thicknesswithin each of the photo-resist recesses 3. It will be appreciated thatthe BCP layer 4 may also be present on top of the photo-resist 2,although this is not shown. In FIGS. 1D and 1E, which show cross-sectionand plan views respectively, the BCP layer 4 has been thermallyannealed. The thermal annealing process causes the BCP deposited withineach of the photo-resist recesses 3 to form distinct domains ofpolymers. A first type A polymer domain 5 is formed as a cylinder withina continuous type B polymer domain 6 within each of the recesses 3.

The cylindrical type A polymer domains 5 are generally centred withinthe recesses 3. However, as shown in FIGS. 1D and 1E, it has beenrealised that the domains 5 may not be accurately centred within therecesses 3. The polymer domains 5 may suffer from random placementerrors within the recesses 3. The placement of each polymer domain 5 isconfined by the resist sidewalls. However, the random nature of theinteractions between polymer chains and resulting phase separation mayresult in placement errors of the polymer domains 5. Any such placementerrors may result from the fact that the energy cost of smalldisplacements is overcome by the thermal energy (kT) present during theannealing process.

The placement of cylindrical A block domains within recesses as guidedby directed self-assembly techniques, and as described above, can besimulated using dynamic density function theory (DDFT) methods. Suchsimulations reveal that the placement error of a cylindrical featurewithin a recess is related to the length of the BCP blocks.

For example, for a system in which the BCP is PS-b-PMMA the placementerror is proportional to the square root of the polystyrene blocklength. The BCP domains can be modelled as a system of springs, in whichthe cylindrical A block domain, which is formed from PMMA, is held inplace by a spring which is comprised of the polystyrene B block domain.

An effective spring constant which models the action of the polystyreneB block domain can be defined as:

K _(spring)=3kT/Nb ²  (1)

where:

-   -   k is the Boltzmann constant;    -   T is the temperature;    -   N is the number of monomer units within the respective BCP block        (i.e. the degree of polymerization); and    -   b is a parameter which represents the length of the monomer        units within the respective BCP block.

It will be appreciated that if the spring constant is increased, thesystem will become more rigid (i.e. stiffer), and the movement of thecylindrical A block domain will be more restricted. This will result inlower placement error. It can be seen from equation (1) that the springconstant can be increased by reducing the block length (i.e. by reducingthe degree of polymerization).

However, it will also be appreciated that the block length should alsobe long enough to induce phase separation, as governed by Flory-Hugginstheory. Further, the total polymer length (i.e. A and B block) should besized appropriately to cooperate with the dimensions of the lithographyfeatures which are intended to be defined (and also therefore with thedimensions of the graphoepitaxy features).

Alternative methods for increasing the effective spring constant of BCPsystem would be to change the stiffness of the polymer blocksthemselves, or to increase the Flory-Huggins interaction parameter.However, either of these methods would involve altering the BCPs usedfrom those which are well understood (e.g. PS-b-PMMA). Further, anyincrease in the Flory-Huggins interaction parameter, or increase inpolymer block stiffness may well introduce significant processingdifficulties, such as, for example, slower kinetics (i.e. slowerself-assembly) and an increased number of defects.

As such, it will be appreciated that using known methods it may not bepossible to control the phase separation to achieve a predictableplacement of each of the polymer domains within the recesses. Therefore,it may not be possible to create graphoepitaxy lithography featuresusing known methods which are positioned with sufficient accuracy tomeet critical dimension (CD) and local critical dimension uniformity(LCDU) requirements.

It is therefore desirable to provide a directed self-assembly methodwhich allows improved placement accuracy. Moreover, it is desirable toprovide such a method whilst allowing well understood and easilyprocessable BCPs, such as, for example, PS-b-PMMA, to be used.

The present invention overcomes the problem which was illustrated inFIG. 1 in which the placement of the polymer domains 5 within therecesses 3 are subject to significant random variation. This problem isovercome in the method illustrated by FIG. 2 by the use of cross-linkingwithin the BCP. Cross-linking reduces the extent to which the polymerchains within the BCP can move randomly. This reduction in movement inturn reduces the extent to which the domains are randomly located withinrecesses. Such a reduction in random location of domains is used toensure that the arrangement of domains is regular and that each domainis accurately positioned within a respective recess.

FIG. 2 shows a process in which a modified BCP is caused toself-assemble. FIGS. 2A and 2B show in plan and cross-sectionrespectively, a substrate 10 on which a layer of photo-resist 11 isprovided. The layer of photo-resist 11 is patterned with a plurality ofcontact hole resist recesses 12.

In FIG. 2C, a self-assemblable A-B block copolymer (BCP) layer 13 hasbeen deposited within the recesses 12. The self-assemblable A-B blockcopolymer is selected to enable a cross-linking between the blocks. Forexample, the BCP may be similar to PS-b-PMMA, modified so as tointroduce a cross-linking group to the PS blocks.

In FIGS. 2D and 2E, the substrate 10 is shown after thermal annealing.The thermal annealing process causes self-assembly of the BCP material.As can be seen within the contact hole resist recesses 12, the BCPmaterial self-assembles to form a domain of A blocks 14 (unhatched) anda domain of B blocks 15 (hatched). The A block domain 14 is in acylindrical arrangement being surrounded by a continuous the B blockdomain 15. The cylindrical A block domain 14 is oriented perpendicularto the substrate 10.

In order to prevent the cylindrical A block domains 14 from beingrandomly located within the respective recesses 12 (i.e. to cause thecylindrical A block domains 14 to be accurately located at the centre ofthe respective recesses 12—as shown in FIGS. 2D and 2E) the BCP iscaused to cross-link gradually, with the cross-linking starting at theperimeter of the recesses 12, and proceeding in a directional mannertowards the centre of the recesses 12, as described in more detailbelow.

In subsequent processing steps (not shown) the domains of A blocks 14can be selectively removed by well known techniques. Such selectiveremoval of A block domains 14 exposes the substrate 10 below the domainsof A blocks 14. However, domains of B blocks 15 will not be removed bythe process which removed A blocks 14, due to the selectivity of theetching process. In this way, it is possible to remove only regions oftype A polymer, with all other areas of the substrate 10 being coveredby either B block domains 15, or photo-resist 11.

The remaining B block features may subsequently be used as a maskdefining openings which can be etched. For example, contact holes maysubsequently be etched in the substrate 10 as defined by the relativelysmall opening presented by the removed A block domains. This processallows a higher resolution to be achieved than could be achieved byconventional photo-resist patterning techniques, the dimensions of thelithographically defined contact hole resist recesses 12 directing theself-assembly of the BCP to create a smaller region of A block domains14.

Selective etching is achieved due the relative susceptibility towardsetching, with the A blocks being relatively prone to etching, while theB blocks are relatively resistant to etching. Selective removal may alsobe achieved, for instance, by selective photo-degradation orphoto-cleavage of a linking agent between blocks of the copolymer andsubsequent solubilisation of one of the blocks. The invention allows forformation, onto substrates, of features which have critical dimensionswhich are smaller than those of the recesses which direct theself-assembly, allowing features of the order of a few nm to be createdwith a smallest lithographically defined recess of the order of a fewtens of nm. For example, the use of a lithographically defined circularrecess having a diameter of about 100 nm may be used to define a contacthole feature having a diameter of about 40 nm. In a further example, theuse of a lithographically defined circular recess having a diameter ofabout 30 nm may be used to define a contact hole feature having adiameter of about 5 nm.

In an embodiment (not illustrated) the etching (or other removalprocess) may etch into the substrate. Following this the type A domainsmay be removed, leaving behind a regularly spaced array of lithographyfeatures formed in the substrate, with a critical dimension which issmaller than the minimum dimension which can be achieved by thephotolithography feature which was used to define the recesses 12.

An example of the process illustrated in FIG. 2 is shown in FIG. 3 inmore detail. FIG. 3A shows a substrate 20 on which a layer ofphoto-resist 21 is provided. A recess 22 is provided at the surface ofthe substrate 20. The recess 22 has side-walls 22 a and a base 22 b. Thephoto-resist layer 21 contains acid molecules 23. For example, the acidmolecules 23 may be produced by a photo-acid generator (PAG) which isprovided within the photo-resist, the photo-resist being a chemicallyamplified photo-resist. The use of chemically amplified photo-resists isknown in the art to increase the sensitivity of the resist to anexposure dose. The acid molecules 23 may comprise, for example, greaterthan or equal to about 1% by weight of the photo-resist layer 21. Theacid molecules 23 may comprise, for example, less than or equal to about20% by weight of the photo-resist layer 21. The PAG may have beenactivated by EUV radiation in an exposure when the recess 22 wasdefined. Alternatively, or additionally, the PAG may be activated by aflood exposure of radiation at a different wavelength, for example UV orDUV radiation.

The photo-acid generator may be selected from known photo-acidgenerators. For example, in an embodiment the photo-acid generator maycomprise onium salts, such as, for example, triphenylsulfonium salts,sulfonium salts, iodonium salts, diazonium salts or ammonium salts. Inalternative embodiments, the photo-acid generator may comprise, forexample, 2,6-nitrobenzyl esters, aromatic sulfonates, sulfosuccinimidesor di-t-butylphenyl iodonium perfluorobutyl sulfonate.

In FIG. 3B, a self-assemblable A-B block copolymer (BCP) layer 24 hasbeen deposited within the recess 22. The BCP is a modified PS-b-PMMA.The PS blocks are modified by the addition of a functional group whichallows the BCP to be cross-linked. For example, the functional groupsmay be acid sensitive cross-linking functional groups.

FIG. 3B′ shows schematically a molecule of a BCP modified so as tointroduce acid sensitive cross-linking functional groups. The moleculecomprises a PS block 24 a and a PMMA block 24 b. The molecule furthercomprises acid sensitive cross-linking functional groups 24 c which areattached to the PS block 24 a.

In an embodiment, the acid sensitive cross-linking functional groups 24c are glycidyl moieties. A small percentage of the glycidyl moieties areincluded within the PS blocks 24 a. This may be achieved, for example,by adding glycidylmethacrylate (GMA) to the PS during synthesis. Theresulting BCP may be referred to as PS/PGMA-b-PMMA. When exposed to anacid, the glycidyl moieties within the PS blocks undergo an acidcatalysed polymerisation reaction. The polymerisation between theglycidyl moieties causes the PS blocks, within which the glycidylmoieties are distributed, to become cross-linked to one another.

It will be appreciated that the cross-linking functional groups may befunctional groups other than glycidyl moieties. For example, thecross-linking functional groups may be epoxide functional groups.

The percentage of glycidyl moieties within the PS blocks may be, forexample, less than about 10% by weight. The percentage of glycidylmoieties within the PS blocks may be, for example, greater than about0.1% by weight. Preferably the percentage of glycidyl moieties withinthe PS blocks is less than or equal to about 2% by weight. Preferablythe percentage of glycidyl moieties within the PS blocks is greater thanor equal to about 1% by weight.

The acid catalysed cross-linking is initiated by the acid molecules 23which are present in the photo-resist layer 21 which were generated inan earlier processing step, as described above.

In FIG. 3C the substrate 20 is placed on a hotplate 25 for thermalannealing. The elevated temperature during the annealing step allows theBCP material to self-assemble to form a domain of A blocks 26 and adomain of B blocks 27. The A block domain 26 is in a cylindricalarrangement being surrounded by a continuous the B block domain 27. Thecylindrical A block domain 26 is oriented perpendicular to the substrate27. The domains 26, 27 may be further processed, for example to removethe A block domain, in further processing as described above withreference to FIG. 2.

The self-assembly process occurs at a rate which depends on theproperties of the BCP material, and other factors such as, for example,the temperature of the annealing process. The annealing temperatureshould be above the glass transition temperature Tg but below the orderdisorder temperature To/d of the BCP.

It is noted that in addition to the A block domain 26 and the B blockdomain 27 there is a further A block domain 26′ surrounding the B blockdomain 27. This domain 26′ consists of A blocks which are in contactwith the side-walls 22 a of the recess 22. The A blocks, due to theiraffinity with the side-walls 22 a tend to flatten against the side-walls22 a. As such, the domain 26′ may not appear as a separate domain and isshown for schematic purposes only. That is, the thickness of the domain26′ in the lateral direction (i.e. the distance it extends from theside-wall 22 a) is minimal. The thickness of the domain 26′ in thelateral direction may, for example, be a few nm (e.g. around 4 nm).

In addition to the self-assembly during the annealing step, the acidmolecules 23 within the photo-resist layer 21 gradually diffuse into theBCP layer 24 from the side-walls 22 a. The gradual diffusion of the acidmolecules 23 into the BCP layer 24 causes the cross-linking to occur ina directional manner. In more detail, the elevated temperature duringthe anneal step (which allows the BCP to self-assemble) increases themobility of the acid molecules 23 within the BCP layer 24. The diffusionrate of the acid molecules 23 at ambient temperatures may besufficiently low that no polymerisation occurs within the BCP, in spiteof the high concentration of free acid molecules 23 within thephoto-resist layer 21 (and consequent high concentration gradientbetween the photo-resist layer 21 and the BCP layer 24). However, at theelevated temperatures during the anneal step the concentration gradientand increased mobility of the acid molecules 23 leads to significantdiffusion.

The diffusion of the acid molecules 23 into the BCP layer 24 causes theglycidyl moieties within the BCP around the perimeter of the recess 22to undergo the acid catalysed polymerisation reaction described above.This causes the PS blocks within the BCP around the perimeter of therecess 22 to be cross-linked. FIG. 3C shows a small number of theglycidyl moieties within the BCP around the perimeter of the recess 22being polymerised (i.e. a small number of the PS blocks around theperimeter of the recess 22 being cross-linked).

The cross-linking between the PS blocks causes the PS blocks to becomemechanically restricted. This has the effect of reducing the length ofthe PS blocks which are free to move so as to adapt differentconformations. This increases the effective spring constant K_(spring)as described above with reference to Equation (1). The increased springconstant causes an effective stiffening in the springs which control theplacement of the A block domain 26. This spring thus stiffening resultsin a reduction of the placement error of the A block domain 26.

The effect of the cross-linking can further be understood by imaginingthe B block domain 27 gradually stiffening as the PS blocks within itbecome more cross-linked. This PS block cross-linking causes a generalincrease of the stiffness of the B block domain 27, which restricts themovement of the cylindrical A block domain 26.

As the annealing process continues, the acid molecules 23 will graduallydiffuse further into the BCP layer 24 within the recess 22, driven bythe acid concentration gradient. As the acid molecules 23 diffusefurther into the BCP layer more glycidyl moieties are caused topolymerise, and more BCP to become cross-linked. FIG. 3D shows a highproportion of the glycidyl moieties within the PS blocks beingcross-linked.

The acid eventually diffuses to the centre of the recess 22, by whichtime glycidyl moieties throughout the PS blocks have become polymerised.The polymerisation of the glycidyl moieties initially around theperimeter of the recess 22, and then gradually towards the centre of therecess 22, is an example of cross-linking in a directional manner.

The slow diffusion rate of the acid within the anneal step, andresulting slow rate of PS cross-linking, coupled with the increasedmobility of the BCP (due to the elevated temperature) allows the BCP toadapt to the gradually increasing stiffness caused by the cross-linking.The gradual diffusion of the acid from the perimeter of the recesstowards the centre thus allows the BCP blocks to alter theirconformation so as to result in a placement of the cylindrical A blockdomains being accurately positioned within the recesses 22. Theplacement errors described above with reference to FIGS. 1D and 1E arethus reduced by the use of cross-linking within the BCPs.

An alternative embodiment is illustrated in FIG. 4. FIG. 4A shows incross-section a substrate 30 which is provided with a recess 31. Therecess 31 is patterned into the surface of the substrate 30. The recess31 has side-walls 31 a and a base 31 b. The recess 31 is circular. Thesubstrate may, for example, be a silicon wafer. The recess 31 may beetched into the surface of the silicon wafer. Alternatively, the recess31 may be etched into a layer which is provided on the surface of thesubstrate 30, for example silicon oxide, or silicon nitride.

In FIG. 4B, a photosensitive layer 32 of material containing aphoto-acid-generator is attached to the side-walls 31 a of the recess.FIG. 4B′ shows schematically a molecule of material which forms thephotosensitive layer 32. Each molecule of the material within thephotosensitive layer 32 comprises a binding group 32 a which binds tothe side-walls 31 a and a photosensitive group 32 b. Suitable materialshaving a photosensitive part and a surface active part are described inUS 2007/0278179, which is herein incorporated by reference. For example,the binding groups may be chloro or alkoxy silanes which bind to siliconoxides surfaces (paragraph [0037]). In an alternative example, thebinding groups may be dienes, alcohols or aldehydes which bind tosilicon surfaces (paragraph [0038]). The photosensitive group 32 b, whenactivated by actinic radiation, produces an acid. The photosensitivegroup 32 b may be referred to as a photo-acid-generator (PAG).

In FIG. 4C, a self-assemblable A-B block copolymer (BCP) layer 33 hasbeen deposited onto the substrate 30. The self-assemblable A-B blockcopolymer is a modified PS-b-PMMA which may be referred to asPS/PGMA-b-PMMA, as described above with reference to FIG. 3.

In FIG. 4D, the BCP layer 33 has been thermally annealed on a hotplate34. The thermal annealing process causes self-assembly of the BCPmaterial in a similar process to that described above with reference toFIGS. 1 to 3. As can be seen within the recess 31, the BCP materialself-assembles to form a domain of A blocks 35 and a domain of B blocks36. The A block domain 35 is in a cylindrical arrangement, beingsurrounded by a continuous the B block domain 36. The cylindrical Ablock domain 35 is oriented perpendicular to the substrate 30. Thephotosensitive layer 32 remains inactive during the annealing andself-assembly process.

In FIG. 4E, a radiation source 37 is provided above the substrate 30.The radiation source may be any suitable form of radiation source. Forexample, the radiation source may be a UV lamp, a laser, an LED or anLED array. Once the self-assembly of the BCP layer 33 is complete thesubstrate 30 is exposed to actinic radiation 37 a emitted by theradiation source 37. The actinic radiation 37 a activates thephotosensitive layer 32. The PAG within the photo-sensitive layer 32generates an acid, which diffuses into the self-assembled BCP layer 33.The acid causes the glycidyl moieties within the B block domain 36 tobegin to polymerise. The polymerisation causes cross-linking between thePS blocks within the B block domain 36. This cross-linking process issimilar to that described above with reference to FIG. 3.

However, in contrast to the process described with reference to FIG. 3,the acid is generated by the photosensitive layer 32 which is attachedto the side-walls 31 a of the recess 31, rather than being providedwithin a resist layer (which is not present in FIG. 4). While the originof the acid differs between the processes shown in FIGS. 3 and 4, theacid is still provided initially at the perimeter of the recess 31,before diffusing towards the centre of the recess 31. This diffusionresults in the polymerisation reaction, and resulting cross-linking,proceeding in a similar fashion for both methods (i.e. from theperimeter towards the centre).

FIG. 4F shows a high proportion of the glycidyl moieties within the PSblocks being cross-linked. The acid eventually diffuses to the centre ofthe recess 31, by which time all of the glycidyl moieties within the PSblocks have become polymerised. This causes the central A block domain35 to be accurately positioned at the centre of the recess 31.

It will be appreciated that the photosensitive layer 32, when attachedto the side-walls 31 a of the recess 31, may also be attached to thebase 31 b of recess 31. For example, the binding group may bind well toboth the side-walls 31 a and the base 31 b and as such may becomeattached to both surfaces during an application process. However, whilea small amount of acid may be generated at the base 31 b of the recess31 as a result of any subsequent exposure to radiation, this does notaffect the progression of the cross-linking from the side-walls towardsthe centre of the recess 31. It is understood that this is a result ofthe quantity of acid generated at the base 31 b of the recess 31 beingsignificantly less than that which is generated at the side-walls 31 a,the side-walls having a larger surface area than the base.

Alternatively, the layer 32 may be arranged to selectively attach toonly the side-walls 31 a, and not to the base 31 b. For example, thebase may be formed from a different material to the side-walls, orprovided with a coating to which the layer 32 does not attach. In anembodiment, the side-walls may be formed from silicon, and the baseformed from a metallic or organic material. The layer 32 may comprise asilane binding group which preferentially binds to the siliconside-walls, and not to the metallic or organic base.

It will be appreciated that the presence of a layer comprising aphotosensitive group (e.g. layer 32) does not prevent the successfuldirected self-assembly of BCP within a recess. The side-wall surfaces,once coated with the layer comprising a photosensitive group, arepreferably not neutral with respect to the BCP blocks. For example,where a PS-b-PMMA BCP is used, the coated side-wall surfaces arepreferably either PS wetting or PMMA wetting, allowing self-assembly tobe driven by the relative affinities of the PS and PMMA blocks for theside-walls, for themselves, and for each other.

Alternative lithography recess geometries are possible beyond thecircular examples discussed above. For example, FIG. 5 shows a substrate40 provided with a photo-resist layer 41 in which linear lithographyrecesses 42 are defined. BCP material within the recesses 42 hasself-assembled to form discrete A block domains 43 and B block domains44. In contrast to the earlier embodiments, the A block domains 43 and Bblock domains 44 within the lithography recesses 42 are shown in alamellar arrangement. The elongate arrangement of recesses 42 guides theself-assembly of the BCP to form B-block domains 44 at the edges of therecesses 42 with a single respective A-block domain 43 running along thecentre of each of the elongate recesses 42. The lamellae of the A-blockand B-block domains 43, 44 are orientated with their planar surfaceslying perpendicular to the substrate 40 and parallel to the recesswalls. Alternatively, there may be a plurality of A-block domains whichare lamellae alternating with B-block domains which are also lamellae.

The cross-linking process described above with reference to FIGS. 2-4may be applied to elongate recesses, such as those illustrated in FIG.5, in order to improve the placement of the self-assembled lithographyfeatures. For example, the A block domains 43 may be accuratelypositioned at the centre of the lithography recesses 42 by the action ofan acid diffusing from the sidewalls towards the centre of the recesses42, causing the BCP material to become cross-linked.

Further alternative lithography recess geometries are possible beyondthe circular and elongate examples discussed above. Any recess geometrywhich promotes self-assembly of BCP may be used for a lithographyrecess. In any such alternative geometry, the cross-linking processdescribed above may be applied in order to improve the placement of theself-assembled lithography features.

In general the dimensions of recesses for use with directedself-assembly of BCPs varies in dependence upon the particular BCPselected. For example, the length of the BCP polymer chains affects therecess dimensions at which phase separation and self-assembly occurs. Ashorter length polymer chain is likely to result in a recess with asmaller dimension being suitable to direct self-assembly of that polymerchain.

In an embodiment the recesses formed on a substrate may have a lateraldimension of less than about 100 nm. The recesses may have a lateraldimension of greater than about 30 nm. If smaller recess dimensions areused than will permit phase separation to occur then the BCP within arecess will not self-assemble.

A dimension of the lithography features (e.g. the A block domains 14;26; 35; 43) formed according to embodiments of the invention may be lessthan about 40 nm. For example, a dimension of the lithography featuresformed (e.g. A block domains) may be greater than about 5 nm.

It will be appreciated that in alternative embodiments differentarrangements of polymer domains may be used. For example, in analternative embodiment which uses the same A-B block copolymer describedabove (PS-b-PMMA), B blocks (e.g. PS) may preferably lie adjacent to theside-walls due to their affinity with the side-walls, while the A blocks(e.g. PMMA) form a central cylindrical domain.

It will be appreciated that the use of resist (also known asphoto-resist) to form the sidewalls of the lithography recesses isintended to be an example, rather than a limiting feature. For example,recesses may be provided by patterning of the substrate itself (forexample as described with reference to FIG. 4), or patterning of a layerdeposited or grown onto the substrate. Alternatively, recesses maythemselves be provided by the self-assembly of a BCP material.

It will be appreciated that initiating cross-linking at recessside-walls is one way of providing cross-linking in a directionalmanner. However, alternative directional cross-linking processes may beused. For example, an acid may be provided on a surface of recess baseat a predetermined location (for example by the inclusion of a PAG atthe predetermined location), causing cross-linking to be initiated atthe predetermined location and proceed away from the predeterminedlocation.

In an embodiment a lithographic tool is arranged carry out annealing ofthe substrate while also providing radiation, for example, as shown inFIG. 4E. The hot plate 34 is an example of heat source which is arrangedto provide heat (to anneal the substrate 30) while the radiation source37 provides radiation 37 a (to initiate cross linking of the BCP layer33). The heat source may be arranged provide thermal energy to a firstsurface of the substrate at the same time as the radiation sourceprovides radiation to a second surface of the substrate.

A lithographic tool as described above may be provided on a wafer trackwithin a semiconductor fabrication plant. For example, the lithographictool may be integrated in to a wafer track which also serves totransport a wafer between lithography apparatus.

The radiation source may be arranged to emit actinic radiation, forexample UV radiation. Alternatively, the radiation source may bearranged to emit EUV or DUV radiation. The radiation source mayirradiate the substrate with a flood exposure i.e. a radiation beamwhich is not patterned. The use of a non-patterned radiation beam allowsany photo-acid generator provided on a substrate surface to beactivated. The wavelength of radiation provided in a flood exposure doesnot determine the minimum feature size which can be formed by processesas described above. Therefore, a flood exposure does not require the useof radiation with an extremely short wavelength (e.g. EUV), as nopattern information is required to be transferred by the exposure.

1. A method of forming at least one lithography feature, the methodcomprising: providing at least one lithography recess on a substrate,the or each lithography recess comprising at least one side-wall and abase, with the at least one side-wall having a width between portionsthereof; providing a self-assemblable block copolymer having first andsecond blocks in the or each lithography recess; causing theself-assemblable block copolymer to self-assemble into an ordered layerwithin the or each lithography recess, the ordered layer comprising atleast a first domain of first blocks and a second domain of secondblocks; causing the self-assemblable block copolymer to cross-link in adirectional manner; and selectively removing the first domain to formlithography features comprised of the second domain within the or eachlithography recess.
 2. The method according to claim 1, wherein thecross-linking is initiated at the at least one side-wall and proceedsaway from the at least one sidewall.
 3. (canceled)
 4. The methodaccording to claim 1, wherein causing the self-assemblable blockcopolymer to cross-link takes place during an annealing process.
 5. Themethod according to claim 1, wherein the block copolymer comprisespolystyrene blocks.
 6. The method according to claim 5, wherein thepolystyrene blocks comprise glycidyl moieties.
 7. The method accordingto claim 6, wherein the polystyrene blocks comprise more than about 0.1%by weight of glycidyl moieties.
 8. The method according to claim 6,wherein the polystyrene blocks comprise less than about 10% by weight ofglycidyl moieties.
 9. The method according to claim 1, wherein causingthe self-assemblable block copolymer to cross-link is initiated by anacid.
 10. The method according to claim 9, wherein the acid is generatedby a photo-acid generator.
 11. The method according to claim 10, whereinthe acid is at least partially generated by a photo-acid generatorduring a lithography exposure.
 12. The method according to claim 10,wherein the acid is at least partially generated by a photo-acidgenerator during a flood exposure.
 13. The method according to claim 10,wherein the photo-acid generator is provided within a resist in whichthe or each lithography recess is formed.
 14. The method according toclaim 10, wherein the photo acid generator is provided on the at leastone side-wall of the or each lithography recess.
 15. The methodaccording to claim 14, wherein the or each lithography recess is formedin the substrate.
 16. The method according to claim 1, wherein the oreach lithography recess is circular.
 17. The method according to claim16, wherein the self-assemblable block copolymer is adapted to form anordered layer having a cylindrical first domain of the first blocks in acylindrical arrangement surrounded by a second continuous domain of thesecond blocks, the cylindrical first domain being oriented perpendicularto the substrate.
 18. The method according to claim 1, wherein theself-assemblable block copolymer is adapted to form a lamellar orderedlayer wherein first domains are lamellae alternating with second domainswhich are also lamellae, the lamellae of the first and second domainsorientated with their planar surfaces lying perpendicular to thesubstrate and parallel to the at least one side-wall.
 19. The methodaccording to claim 17, wherein the cross-linking is initiated at the atleast one side-wall and proceeds towards the centre of the recess.20.-22. (canceled)
 23. A method of forming at least one lithographyfeature on a substrate, the substrate comprising at least onelithography recess, the or each lithography recess comprising at leastone side-wall and a base, with the at least one side-wall having a widthbetween portions thereof, the method comprising: providing aself-assemblable block copolymer having first and second blocks in theor each lithography recess; causing the self-assemblable block copolymerto self-assemble into an ordered layer within the or each lithographyrecess, the ordered layer comprising at least a first domain of firstblocks and a second domain of second blocks; causing theself-assemblable block copolymer to cross-link in a directional manner;and selectively removing the first domain to form lithography featurescomprised of the second domain within the or each lithography recess.24. A lithographic tool comprising: a heat source configured to anneal asubstrate; and a radiation source configured to irradiate the substrateduring annealing. 25.-26. (canceled)